Polyphosphate Attenuates Oxidative Stress to Support Temperature Adaptability in Hot Spring Cyanobacteria.

Thermophilic cyanobacteria successfully colonize thermal gradients within hot springs, implying evolved mechanisms to cope with temperature-induced oxidative stress. Although polyphosphate (polyP) is known to contribute to oxidative stress resistance, its specific role in thermophilic cyanobacteria remains poorly understood. To address this, this study established a temperature gradient (30-70 °C) and used phloretin (polyP synthesis inhibitor) plus exogenous polyP to investigate polyP metabolism, redox homeostasis, photosynthetic function, and growth of Thermosynechococcus sp. FJSJ-1 from hot spring. The results show that temperature fluctuations specifically induce polyP accumulation, whereas inhibiting polyP synthesis sharply elevates reactive oxygen species (ROS) and overloads intrinsic defenses including superoxide dismutase, catalase, glutathione, and heat shock proteins. Crucially, exogenous polyP rescued phloretin-induced oxidative damage and growth inhibition. PolyP mitigates oxidative damage not by direct ROS scavenging but by integrating and reinforcing endogenous antioxidant network. This protective effect in turn safeguards photosystem II from oxidative attack, thereby preserving photosynthetic pigment stability, phycobiliprotein content, and electron transport efficiency. Taken together, polyP contributes to temperature adaptability in Thermosynechococcus sp. FJSJ-1 by coordinating antioxidant defense. This study elucidates a key molecular strategy for thriving across temperature ranges in geothermal ecosystems, advancing microbial adaptation knowledge and providing a theoretical basis for engineering thermotolerant strains for bioremediation and biofuel production.